Many mobile computing and communicating devices rely upon standard battery cells for providing power on which to operate. Though disposable battery cells, such as alkaline cells, are a well-known and reliable technology, it is common in such mobile devices to employ rechargeable battery cells. These rechargeable batteries depend on a number of known cell types, including Ni-Cad, Ni-MH, and Li-Ion cells. All these cells are known to those of skill in the art, as are some of their deficiencies. One of the known deficiencies of the above mentioned rechargeable battery cells is related to the fact that each battery has a finite life span that can be measured in terms of recharge cycles. The process of charging and discharging the cell damages the cell's charge storage capabilities, causing the stored potential, which is typically measured in mA-hours, to decrease over the life of the battery. As the ability to store charge decreases, so does the battery's utility. The life of the battery can be drastically curtailed by improperly charging, or over discharging the battery. As a result of these deficiencies, it is crucial that a user be able to determine the capacity of a battery both prior to and during the usage.
A state of the art technique for battery capacity reporting relies on the coulomb counter. The principle of operation involved in coulomb counting is computing the difference between the coulombs injected into a battery and the coulombs taken out of the battery. The capacity of the battery is then reported by comparing the coulomb count relative to a reference coulomb count value that corresponds to maximum battery capacity. For instance, if the coulomb count of a battery is half of the reference value, the battery capacity is reported to be 50 percent. Although the coulomb counter addresses battery capacity reporting, it may have several problems. First, the reported capacity may not be meaningful if an accurate reference coulomb count value corresponding to maximum battery capacity is not known. Furthermore, with a coulomb counter it may be difficult to keep an accurate reference coulomb count, particularly when battery capacity decreases over the lifetime of the battery. Further still, with a coulomb counter it may be necessary to know the current battery capacity before beginning the coulomb count.
A limitation of the coulomb counting principle is that it may not be applicable to reporting the capacity of a battery of initially unknown battery capacity: if the capacity of a battery is to be reported using the coulomb count system and method, the battery may have to be taken from its unknown capacity state to either a fully charged 100 percent battery capacity state or to a fully discharged 0 percent capacity state before the coulomb count can be used. Because the state of the battery is unknown at a certain point, the only way to charge the battery to 100% capacity is to constantly provide charge over an extended length of time. This can result in an overcharging of the cell, which is known to damage the storage capability of the cell. Conversely, to guarantee that the cell is at 0% capacity, the cell must be completely discharged. Rechargeable batteries are possibly permanently damaged by being overly discharged.
Further practical limitations exist with coulomb counting techniques. In practice, coulomb counting works by applying integration over time. The presence of an offset in a coulomb counter may result in the inaccuracy of the coulomb count. This applies even to batteries with an assumed initially known battery capacity, and is compounded with every recharge cycle. This may be especially true if the battery needs to be used for a long period of time between opportunities to reset the coulomb counter. For instance, in a battery that needs to be used for 3 weeks between charges, even small offsets with each charge cycle may accumulate to become large inaccuracies in reported capacity.
Other known existing techniques of battery capacity reporting are primarily based on measuring battery voltage.
Batteries have known characteristic charge and discharge curves. FIG. 1 illustrates a charge curve model 130 and a discharge curve model 140 for a battery. These curves relate battery voltage 110 to capacity percentage 120 for a rechargeable battery. Battery capacity percentage 120 is related to battery voltage 110 in either a discharging state, shown by the discharge curve model 140, or the charging state shown by the charge curve model 130. Illustrated is a multiplicity of points such as point 132 on the charge curve model 130 and point 142 on the discharge curve. Interpolation can be used to provide capacity values 120 for voltages 110 that lie between points for which values are known. In reference to FIG. 1, the relationship between battery voltage 110, battery charge state and capacity 120 is illustrated by two curve models 130,140. The first curve model 130 corresponds to a positive battery charge current or battery charging state, and the second curve model 140 corresponds to a negative battery charge current or battery discharging state.
When the battery is in a charging state, a charge curve corresponding to the charging state is utilized. When the battery is in a discharging state, a discharge curve corresponding to the discharging state is utilized. The charge and discharge curves are such that given a battery voltage value and a charge curve or a discharge curve, it is possible to obtain a corresponding capacity value from the curves.
Though it is possible to determine the capacity of a battery by measuring the voltage of the battery and examining the curves, it should be noted that the existence of two distinct curves presents a problem. For example, when a battery voltage is 3.8 V and a power source is plugged into the battery at this time, according to the discharge state curves, there is an abrupt drop of the reported battery capacity from 52% to 17%. The reported result is not correct. Actually, a battery enters a transition phase P1 from discharging to charging when a power source is plugged in while the battery is discharging. After the transition phase P1, the battery goes into the charging state. Similarly, when a power source is removed while charging a battery, for example, at a battery voltage 3.9V, there is an abrupt rise of the reported battery capacity from 49% to 71% based on the charging curve and the discharging curve of FIG. 1. Actually, a battery enters a transition phase P2 from charging to discharging when a power source is removed while charging the battery. After the transition phase P2, the battery goes into the discharging state. Under the above circumstances, the reported result will not be correct if the discharging curve and the charging curve of FIG. 1 are used to report the battery capacity in the transition phases.